the application and properties of ati nushield™ borated stainless steels

8/10/2019 The Application and Properties of ATI NuShield Borated Stainless Steels

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ATICORROSIONCONFERENCE.COM | CORR OSION SOLUTIONS CONFERENCE 2011 PROCEEDINGS 297

PAPER 7D

The Application andProperties of ATI NuShieldBorated Stainless Steels

CharlesStinnerManager, Market and Project Development

ATI Nuclear Energy

Technical and Commercial Center

1300 Pacific Ave

Natrona Heights, PA 15065

USA

T: 724-226-6173

E: [email protected]

Tony DenardoProduct Development Metallurgist

ATI Allegheny Ludlum

1300 Pacific Ave

Natrona Heights, PA 15065

USA

T: 724-226-6317

E: [email protected]

Biography

Charles Stinner is the Manager of Market

and Project Development at ATI Nuclear

Energy. His responsibilities include research

and development of new products as well

as business development of the Electrical

Energy market. His current area of focus is

development of materials for the Nuclear

Energy market as well business development

of nuclear energy accounts. Mr. Stinner

received his PhD in Metallurgical and

Materials Engineering from the University of

Pittsburgh.

Abstract

ATI has begun production of a family of borated

stainless steel products, being branded as ATI

NuShield Borated Stainless Steel. These

products use powder metallurgy processing

which imparts a homogenous microstructure

resulting in properties superior to cast and

wrought products. This paper discusses the

application and properties of these products,

with emphasis on ATI 304B7P/M Alloy.

Keywords

ATI NuShield

Borated Stainless Steel

304B7

spent fuel

neutron absorption

spent fuel rack

neutron attenuation

enriched boron

Introduction

Commercial nuclear reactors rely on the fission

of uranium and/or plutonium compounds to

produce radiation that is converted into heat.

Fission in power reactors is the process where

fissile isotopes, mostly 235U and 239Pu, absorb

a neutron and split into two or more nuclei,

releasing energy, gamma radiation, and free

neutrons. The free neutrons may later be

absorbed by other fissile atoms triggering

further fission events, which in turn release

more neutrons. Control of this chain reaction

is the key to safely harnessing nuclear energy

for power production.

Control of the nuclear chain reaction

is possible by affecting the energy and

availability of neutrons. Within nuclear

reactors, neutron moderators such as water or

graphite are used to reduce the velocity of fast

neutrons such that they become lower energy

thermal neutrons, which are more likely to be

absorbed and create fission events. Similarly,

neutron absorbers may be used to reduce the

number of neutrons available to participate

in fission events, which slows or prevents

the nuclear chain reaction from proceeding.

Neutron absorbers are used in control rods

within the fuel assemblies to control the rate

of power production, or to halt the fission

process during shutdown. Neutron absorbersare also a key component for handling and

storing used fuel, where they are used to

isolate the fuel allowing for the slow decay of

heat and radiation.

The effectiveness of neutron absorbers is

characterized by their neutron cross-section,

which is measured in barns. One barn is equal

to 10-28m2, which is approximately the cross-


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The Application and Properties of ATI NuShield Borated Stainless Steels

CORROSION SOLUTIONSCONFERENCE 2011 PROCEEDINGS | ATICORROS IONCONFERENCE .COM298

PAPER 7D

sectional area of a uranium nucleus. The

higher the neutron cross-section of a given

atom, the greater is the ability of the atom

to absorb a neutron. Table 1 gives a list of

elements along with their neutron absorption

cross-sections [1]. Due to cost and availability,

the most common neutron absorbing elementused today is boron. Natural boron contains

approximately 19.9 atomic percent 10B, which

is the isotope responsible for absorption of

thermal neutrons. Boron compounds enriched

in 10B are available commercially, but are not

commonly employed due to the very high price

of this raw material.

Boron may be used in many forms,

including elemental powder, boron carbidepowder, boron carbide in metal matrix

composites, boron carbide in ceramics, or

as a boride rich phase in stainless steels.

Most boron containing materials require

physical containment or attachment, and are

not suitable for structural members due to

strength and ductility issues. Borated stainless

steels (BSS) have a strength and ductility

advantage over other materials, which makes

them uniquely suited to be used for structural

application such as spent fuel racks.

Used Nuclear Fuel

In a commercial nuclear reactor, power is

generated through the transfer of heat to a

heat transfer medium, such as borated water,

which is used to produce steam that is used

to turn a turbine-generator set. The equipmentthat is used to convert heat into steam can

differ depending on the design of the reactor.

While there have been many different nuclear

reactor designs deployed around the world,

the most common commercial nuclear

reactors in service or under construction are

the pressurized water reactor (PWR) and the

boiling water reactor (BWR).

In these reactor designs, and most others,

power is produced through the fission of

uranium enriched in the fissionable isotope235U. An exception is the AECL designed Candu

reactor, which uses natural uranium (un-

enriched) for fuel. The uranium fuel, in the form

of UO2, is contained within seamless zirconium

tubes called fuel rods. The fuel rods are placed

into fuel assemblies typically containing 50-

264 fuels rods each depending on the type

and size of the reactor core. A PWR will have

120-300 fuel assemblies per core. A BWR fuel

assembly is smaller than that of a PWR, and

will have up to 370-800 assemblies per core2.

The fuel assemblies are arranged within the

reactor core such that the burn-up efficiency

of the fuel in the fuel rods is maximized. After

approximately 12-24 months, nuclear fuel

must be replaced by fresh fuel due to the build-

up of fission products that absorb neutrons.

Typically, one third of the fuel assemblies for

a PWR, and one quarter of the assemblies of a

BWR, are replaced during a refueling outage.

The disposition and storage of this spent fuel

is one of the most challenging aspects of the

operation of a commercial nuclear power plant.

This spent fuel must be carefully contained

as it continues to emit dangerous levels of

radiation and heat for many years.

During a refueling outage, used fuelassemblies are removed from the reactor core

and placed in racks within a spent fuel pool. The

spent fuel pool is a stainless steel lined tank

typically containing a minimum of 6.1 m (~20')

of water (see Figure 1). The water is used to

shield the environment from gamma radiation

and also provide a heat transfer medium to

cool the fuel assemblies. The used fuel will

remain in the spent fuel pool at least until it

Table 1.Neutron cross-section of selected

elements.

Element Neutron A bsorption

Cross-section (Barns)

B (natural) 764

10B 3835

11B 5.5 x 10-3

Cd 2520

Dy 940

Er 159

Eu 4565

Hf 104

In 194

Ag 63.3

Gd 48890

Ni 1.15

Fe 2.56

Zr 0.185

Table 2.Half-life of some common isotopes

found in spent nuclear fuel.

Isotope Half Life

Fission Products

129I 1.57 x 107years

131I 8 days

134Cs 2 years

137Cs 30.1 years

90Sr 28.5 years

99Tc 4.2 x 106years

144Ce 284 days

147Sm 1.06 x 1011years

144Nd 2.29 x 1015years

Actinides

239Pu 24,000 years

241Pu 14.4 years

234U 2.45x105years

235U 7 x 108years

236U 6561 years

238U 4.47 x 109years

241Am 433 years

237Np 2.14 x 106years

Figure 1. Spent fuel storage pool.


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The Application and Proper ties of ATI NuShield Borated Stainless Steels PAPER 7D

has lost enough heat to be safely transferred

to another facility. Typically, a minimum time

of one year is required [2]. In the US and other

countries that dispose of used fuel after one

cycle, spent fuel remains in pools at the reactor

site until the pools near their capacity. The

spent fuel is then transferred from the spentfuel pool to on-site dry storage in casks. O ther

countries will store spent fuel in pools for an

extended period and then transfer the spent

fuel to a reprocessing facility.

It is important to note that used fuel contains

a significant amount of radioactive isotopes,

some with very long half-lives. Typical used

fuel is about 95% 238U, 1% 235U that has not

undergone fission, about 1% plutonium and

3% of various fission products. Some fission

products, such as isotopes of iodine and

cesium, have relatively short half-lives but

are highly radioactive. These isotopes are

responsible for most of the heat generated by

used fuel. Uranium and plutonium, along with

the minor actinides americium, neptunium,

and californium have lower radioactivity, but

have extremely long half-lives. The half-lives of

some of the isotopes that make up spent fuel

are summarized in Table 2[3].

Due to the large amount of fissile materialcontained in used fuel, there is a potential

for criticality to be reached, resulting in an

uncontrolled reaction and potential meltdown.

In order to reduce the chances of criticality, the

fuel assemblies are isolated through the use

of neutron absorbing materials. These may be

mechanically attached to the support structure,

or in the case of borated stainless steels, used

as the structural support material. Borated

stainless steels are used as the material of

construction for racks and baskets in the spent

fuel pools and casks used for transport and

storage, and as neutron absorbing inserts in

some designs.

General Characteristics of

Borated Stainless Steels

Borated stainless steels (BSS) are covered by

the specification ASTM A887-89 [7]. The grade

of borated stainless is characterized by itsboron content, per Table 3. The compositions

are similar in many respects to 304 stainless

steels, with the main exception of the high boron

content. The alloys also have higher nickel

content. The increase in nickel is intended to

compensate for the effects of boron on the

properties and micro-constituents. Nickel is

added to improve ductility and workability.

An optical micrograph of ATI 304B7P/M is

shown in Figure 2. The microstructure consists

of chromium rich boride particles (darker

phase) in an austenite matrix. The boride

phase for similar alloys was researched by

Goldshmidt[4,5]and others [6]and was found to

Figure 2. Optical (a) and SEM micrograph (b) illustrating morphology and distribution of boride precipitates in ATI 304B7P/M alloy.

a b

Table 3.Composition limits for borated stainless steels per ASTM A877-89[7].

UNS Type C Mn P S Si Cr Ni B Other

S30460 304B 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 0.20-0.29 0.10N

S30461 304B1 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 0.30-.049 0.10N

S30462 304B2 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 0.50-0.74 0.10N

S30463 304B3 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 0.75-0.99 0.10N

S30464 304B4 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 1.00-1.24 0.10N

S30465 304B5 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 1.25-1.49 0.10N

S30466 304B6 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 1.50-1.74 0.10N

S30467 304B7 0.08 2.00 0.045 0.03 0.75 18.00-20.00 12.00-15.00 1.75-2.25 0.10N

CONCENTRATIONS ARE THE MAXIMU M, UNLESS A RANGE OR MINIM UM IS INDICATED.COBALT CONCENTRATION SHALL BE 0.2 MAX , UNLESS A LOWER CONCENTRATION IS AGREED UPON BETWEEN THE PURCHASER AND THE SU PPLIER.


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PAPER 7D

have the approximate proportions of Cr:Fe:Ni

of approximately 11:11:1 with an orthorhombic

crystal structure of the type Cr2B.

The precipitation of chromium rich borides

locally depletes the surrounding austenite

of chromium. This degrades the corrosion

resistance somewhat, especially in the presence

of chlorides. The minimum chromium content is

raised so that depletion effects are reduced.

The carbon content is typically kept below

0.03 wt% maximum in order to avoid forming

chromium carbides, which would further

deplete the austenite of chromium resulting in a

further decrease in corrosion resistance. These

alloys are generally less resistant than typical300 series stainless steels, but have good

resistance in the spent fuel pool environment.

Research has shown that alloys produced

using the powder metallurgy (P/M) process

have generally superior corrosion resistance

to ingot cast and wrought materials of similar

composition [8]. Some corrosion testing results

for ATI 304B7P/M in a simulated spent fuel

pool environment may be seen in Table 4.

The mechanical properties of borated

stainless steels as specified by ASTM A887

may be seen in Tables 5 and 6. The properties

for P/M alloys are given as Grade A and

those of ingot cast and wrought products are

designated Grade B. The higher values for

the minimum properties reflect the improved

performance of the P/M material.

Table 5.Mechanical testing requirements of borated stainless steels per ASTM A887-89 [7]. Grade A: P/M produced, Grade B: cast and wrought.

UNS

Designation

Type Grade Tensile Strength, min Yield Strength, min Elongation, min Hardness, max

MPa ksi MPa ksi % Brinell Rockwell B

S30460 304B A 515 75 205 30 40.0 201 92

B 515 75 205 30 40.0 201 92

S30461 304B1 A 515 75 205 30 40.0 201 92

B 515 75 205 30 35.0 201 92

S30462 304B2 A 515 75 205 30 35.0 201 92

B 515 75 205 30 27.0 201 92

S30463 304B3 A 515 75 205 30 31.0 201 92

B 515 75 205 30 19.0 201 92

S30464 304B4 A 515 75 205 30 27.0 217 95

B 515 75 205 30 16.0 217 95

S30465 304B5 A 515 75 205 30 24.0 217 95

B 515 75 205 30 13.0 217 95

S30466 304B6 A 515 75 205 30 20.0 241 100

B 515 75 205 30 9.0 241 100

S30467 304B7 A 515 75 205 30 17.0 241 100

B 515 75 205 30 6.0 241 100

Table 4.Modified ASTM G 48 Practice B crevice test and Practice A pitting testresults for ATI 304B7P/M alloy. Solution: 10 ppm Cl- (as NaCl) and 12,000

ppm boric acid solution, exposure time: 72 hours, temperature: 24C (75F).

Modified G48 Practice A

Sample Code Test Temperature

(C)

Weight Loss

(g )

Weight Loss

(g/cm2)

Deepest Pit

and Location

420-A1 24 0.0003 0.000009 None

420-A2 24 0.0003 0.000010 None

Modified G48 Practice B

Sample Code Test Temperature

(C)

Weight Loss

(g )

Weight Loss

(g/cm2)

Deepest Pit

and Location

420-B1 24 0.0004 0.000013 None

420-B2 24 0.0002 0.000010 None


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ATI NuShield Products

The careful, highly specialized processing of

ATI NuShield products allows for superior

properties over competing products.

ATI begins wi th powder metal lurgy processing

which imparts a homogenous microstructure,leading to higher ductility and toughness,

along with improved neutron absorption

performance. The material may be used as-

consolidated as a near net shape, or further

processed into plate, sheet or strip.

Powder Metallurgy

Processing

The powder metallurgy processing used

to produce ATI NuShield products is

performed at the production facilities of ATI

Powder Metals. The process begins with

induction melting and inert gas atomization.

Similar to conventional metallurgy, the

composition is attained in the melt furnace,

and not via elemental blending of powders.

Once chemistry is confirmed and refining

is complete, rather than pouring the molten

metal into ingots or molds, the material is

diverted through a nozzle where the stream is

impinged by high velocity inert gas (typically

nitrogen or argon), rapidly solidifying

the material and eliminating segregation

that would occur with conventional ingot

metallurgy. The high-purity, homogenous,

and spherical powder particles (see Figure 3)

are collected at the bottom of the chamber for

subsequent processing. Again, each powder

particles shares a common chemistry, and

can be thought of as a micro-ingot.

Following atomization, which yields a wide

size range of powder particles, the material

is classified to a desired particle size by

way of sieving. This screening operation is

performed in clean room conditions under apositive pressure of filtered air. Next, the yield

of multiple heats are blended together, both to

homogenize the particle size distribution and

normalize any compositional variations. The

former is of utmost importance for production

of consolidated product, as it influences

packing density in the containers and

consequently distortion during consolidation.

Once the powder has been screened,

Table 6.Impact testing requirements of borated stainless steels per ASTM A887-89 [7].

Grade A: P/M produced, Grade B: cast and wrought.

UNS

Designation

Type Grade Charpy V-Notch Energy, min

J ft-lb

S30460 304B A 65 88

B 40 54

S30461 304B1 A 60 81

B 35 47

S30462 304B2 A 48 65

B 16 22

S30463 304B3 A 38 52

B 10 14

S30464 304B4 A 30 41

B

S30465 304B5 A 23 31

B

S30466 304B6 A 16 22

B

S30467 304B7 A 10 14

B

Figure 3. Scanning electron microscope (SEM) micrograph of inert gas atomized powder.


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PAPER 7D

Figure 4. Schematic of powder metallurgy processing.

Melting & Atomization Screening Blending Loading Hot Isostatic Pressing

Figure 5. Microstructures of borated stainless steel produced by (a,b) ingot metallurgy, cast and wrought structures and (c,d) powder metallurgy.

a b

c d

Cast + Wrought 1.1% Boron Cast + Wrought 1.65% Boron

P/M with 1.1% Boron P/M with 1.85% Boron


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blended, and fully classified, custom-

engineered containers are fabricated for

optimum integrity and loaded with the powder

on vibratory tables to maximize packing, again

to control the extent of shrinkage and distortion

that occur during consolidation. The final step

in the process is hot isostatic pressing (HIP),

where the compacts are loaded into ATI Powder

Metals autoclave, and heat and pressure

are simultaneously applied per established

procedures. A processing schematic is shown

in Figure 4. HIP consolidation achieves full

theoretical density of the P/M material. The

consolidated material, termed a compact, may

be used as a near net shape or can be further

worked at ATI manufacturing operations to

produce the final product.

Properties of ATI

NuShield Products

A comparison of the microstructure and

properties of borated stainless steel produced

by powder metallurgy and ingot cast +

wrought processes is given in Figure 5.

The microstructure shown in Figure 5d is of

mill produced ATI 304B7 flat-rolled sheet

product. All other materials shown in this

figure were produced in 20 kg heat lots and

processed at ATI production facilities. The

fine distribution of borides of the P/M product

leads to the higher ductility and toughness

noted in Table 7.

Typical mechanical properties of ATI

304B7P/M are given in Table 8. This grade

is currently available in flat rolled products

from ATI Allegheny Ludlum as plate, sheet, and

strip. Other product forms and compositions

are available on request.

Neutron Absorption

Neutron absorption testing of ATI 304B7

P/M was performed on a university researchreactor. Several samples were tested in order

to determine the ability of the ATI 304B7

P/M material to attenuate neutrons. Two

control samples of commercially available

type 304L were also run for comparison. The

results of the testing are shown in Table 9.

The ratio I/Io is a measure of the intensity of

the transmitted neutron beam to the incident

beam with no sample present. It is noted that

significant neutron attenuation was achieved

for all of the borated stainless steel samples.

Very few neutrons were able to pass through

the discrete plate samples labeled as PMP-1

and PMP-2.

Fabrication

Borated stainless steels have higher strength

and lower ductility than 300 series stainless

steels and special precautions are needed to

prevent cracking or degradation of properties.

ATI NuShieldalloys are generally weldable

using standard techniques. However, some

degradation in properties of the heat affected

zone should be expected. The use of AWS ER

308/309 filler metal is recommended for best

results. Low heat inputs should be used tominimize base metal dilution. A photograph of

3mm thick sheet of ATI 304B7 P/M that has

been butt welded using MIG with ER 308 filler

wire and low heat input may be seen in Figure

6. The weld joint was able to be bent through

approximately 145 without cracking.

ATI NuShieldproducts can be cold formed

by bending although the angle and radius may

be limited. The edges should be ground and

Table 7.Property comparison between P/M and cast + wrought products. All materials were produced in 20 kg heat lots and processed at ATI facilities.

Identification UTS MPa (ksi) YS MPa (ksi) Elongation % Impact J (ft-lb)

ATI 304B7 P/ M

1.8% Boron738 (107) 338 (49) 25 22 (16 ft-lb)

C&W 304B61.65% Boron

648 (94) 351 (51) 11 9 (6.5 ft-lb)

P/M 304B5

1.4% Boron772 (112) 303 (44) 35 38 (28 ft-lb)

C&W 304B5

1.4% Boron634 (92) 303 (44) 18 14 (10 ft-lb)

P/M 304B4

1.14% Boron689 (100) 283 (41) 35 65 (48 ft-lb)

C&W 304B4

1.1% Boron710 (103) 462 (67) 21 18 (13 ft-lb)

ASTM 887-89

304B7 Grade A

515 (75) 205 (30) 17 14 (10 ft-lb)

ASTM 887-89

304B7 Grade B

515 (75) 205 (30) 6

GRADE A PROPERTIES ARE FOR P/M PRODUCED MATERIAL.

GRADE B ARE FOR CAST + WROUGHT.PROPERTIES LISTED ARE AVERAGE OF TESTS ON 6MM -THICK MILL-PRODUCED PLATE.PROPERTIES ARE FOR LAB ORATORY PRODUCED MATERIAL.


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PAPER 7D

free of area of stress concentration such as

gouges, burrs, dents, etc. This is especially

true of the grades with higher boron content,

which tend to be highly notch sensitive. Some

bend testing data for ATI 304B7P/M may be

seen in Figure 7 and Table 10.

Hot working should be performed between

927-1,093C (1,700-2,000F). To restore

properties, hot or cold worked products

should be annealed at a temperature range of

1,038-1,093C (1,900-2,000F) followed by

rapid cooling.

ConclusionBorated stainless steels are very effective

neutron absorbers and possess properties

that allow these alloys to be used for structural

applications such as spent fuel racks and

baskets. ATI NuShieldborated stainless steel

products are made using P/M processing,

which results in a very fine, homogenous

distribution of boride precipitates. The

homogenous microstructure leads to high

ductility and toughness, improved neutron

absorption, and better corrosion performance

than conventional ingot cast products. ATINuShield alloys are available as flat rolled

products from ATI Allegheny Ludlum.

References

1. ASTM C1233-98, Standard Pract ice for

Determining Equivalent Boron Contents

of Nuclear Materials, American Society

Figure 6. Welded ATI 304B7P/M sheet that was bent through ~145 without cracking.

Table 9.Results of neutron attenuation testing. I/Io is the ratio of the transmitted beam to the incident beam.

Sample Thickness mm (in) Boron wt% Areal Density (g/cm2) I/Io

304-1 2.67 (.105) 0.0001 0.00038 0.938

304-2 3.30 (0.130) 0.0001 0.00047 0.867

S-1 2.72 (0.107) 1.84 0.007 0.258

S-2 3.30 (0.130) 1.80 0.008 0.217

PMP-1 6 (0.236) 1.80 0.015 0.098

PMP-2 10 (0.394) 1.80 0.026 0.063

Table 8.Typical mechanical properties of ATI 304B7P/M alloy.

Identification UTS MPa (ksi) YS MPa (ksi) Elongation % Impact J (ft-lb)

Sheet 758 (110) 335 (48) 19 N/A

Plate 738 (107) 338 (49) 24 22 (16 ft-lb)

ASTM 887-89

304B7 Grade A

515 (75) 205 (30) 17 14 (10 ft-lb)

TYPICAL EXPECTED PROPERTIES FOR SHEET 2 MM THICKNESS.TYPICAL EXPECTED PROPERTIES FOR HOT-ROLLED PLATE BETWEEN 5MM AND 12MM THICKNESS.


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for Testing and Materials, 2010.

2. International Atomic Energy Agency,

Storage and Disposal of Spent Fuel

and High Level Radioactive Waste,

Fact Sheet.

3. G. Radulescu, I.C. Gauld, and G. Ilas,

Scale 5.1 Predictions of PWR Spent

Nuclear Fuel Isotopic Compositions,

ORNL, ORNL/TM-2010/44, March

2010.

4. H.J. Goldschmidt, Effects of Boron

Additions to Austenitic Stainless Stee ls,

Journal of the Iron and Steel Insitute,

209, 1971.

5. H.J. Goldschmidt, Effects of Boron

Additions to Austenitic Stainless Stee ls.

Part II Solubility of Boron in 18% Cr,

15% Ni Austenitic Steels, Journal of the

Iron and Steel Insitute, 209, 1971.

6. E.A. Loria, and H.S. I saacs, Type

304 Stainless Steel with 0.5% Boron

for Storage of Spent Fuel, Journal of

Metals, December, 1980.

7. ASTM A887-89, Standard Speci fication

for Borated Stainless Steel Plate, Sheet,

and Strip for Nuclear Applications,

American Society for Testing and

Materials, 2010.

8. T.E. Lister, R.E Mizia, A.W. Erikson, and

B.S. Matteson, General and Localized

Corrosion of Borated Stainless Steels,

NACE, Corrosion 2008, 2008.

n n n

Table 10.Bend testing results for ATI 304B73 mm (0.125") thick sheet.

Test Specimen Transverse Longitudinal

12.7 mm radius to 180 Pass Pass

6.4 mm radius to 180 110 Pass

6.4 mm radius to 90 Pass Pass

3.2 mm radius to 90 83 77

Figure 7. 3 mm (0.125") thick ATI 304B7P/M sheet bent to 90 at a radius of 6.4 mm (0.25")

and 12.7 mm (0.5").

the application and properties of ati nushield™ borated stainless steels

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